When it happens to humans, it's called 'couvade syndrome' as if it was some sort of illness. But the bit of extra weight an expectant dad puts on during his mate's pregnancy may not be a sick psychosomatic reaction at all.
A group of primate researchers who study two very unique species — common marmosets and cotton-top tamarins — noticed that males mated to pregnant females gained between 2-20% extra weight during her pregnancy.
The only other primates species reported to gain weight during their mate's pregnancy are humans.
In one study of couvade, derived from the French ‘to incubate or hatch,' 47% of the 81 expectant fathers in the study experienced significant weight gain during their partner's pregnancy. They also reported nausea, headaches backaches, restlessness, and anxiety. The medical and psychological community believe these to be sympathetic reactions to a mate's pregnancy without a biological cause, but perhaps there is one.
According to primatologist Toni Ziegler and co-authors, expectant fathers might be preparing for the energetic cost of fatherhood by gaining weight during their mate's pregnancy. But why?
Tamarins and marmosets are unique among primates for their high degree of male parental care.
Females give birth to twins that can weigh as much as 20% of their body weight, which for me, would be like giving birth to not one, but TWO babies that weigh 12 pounds! That's a lot of weight to haul around. Primates carry their babies with them either on their bellies or their backs wherever they go. Sure, a few oddballs (bushbabies) stash them in tree holes, but the vast majority of primates carry the babes more or less constantly. Baby-wearing is not some new trend among celebrity people, it is an old, old custom.
What makes tamarins and marmosets especially unique is that the dads carry the babies — not the moms. Moms just nurse them. It's the dads who do the heavy lifting. Thus, it makes good evolutionary sense for them to bulk up. They'll be burning a lot of extra calories packing around twins wherever they go so having a good supply of fat laid down in advance helps. Moms gain weight too, of course. Nursing is also energetically costly.
Pregnancy weight gain: marmoset and tamarin dads show it too. (2006) Ziegler et al.
P.S. Dear Mr. Field Notes, if you're reading this it's okay to hog the peanut butter cookies, just save some for me and expect to pitch in with baby carrying =D